1. Field of the Invention
The present invention relates to an inspection method of a mask pattern formed on a photo mask for exposure for use in manufacturing a semiconductor device, particularly to an inspection method of a mask pattern for more strictly judging whether a defect of the mask pattern found by inspection is allowable.
2. Description of the Related Art
In recent years, semiconductor devices such as LSI have been formed to be finer, and accordingly a size of a mask pattern formed on a photo mask for exposure for use in manufacturing the semiconductor device also needs to be below 1 μm. Moreover, phenomena in which the pattern cannot be transferred onto a wafer as designed, that is, optical proximity effects (OPE) have also clearly existed. Therefore, a technique of using the corrected mask pattern to finish the transferred shape in accordance with a desired design pattern, that is, optical proximity correction (OPC) has been required. When this technique is introduced, it is possible to suppress a CD (Critical Dimension) variation on the wafer. Thereby, even a finer pattern can faithfully be finished on the wafer as designed. As a result, conversely, a necessary shape as the mask pattern actually formed on the photo mask for exposure is largely different from a desired pattern shape (design value) on the wafer. In this photo mask for exposure, it has become difficult to inspect defects of the mask pattern.
In an inspection apparatus for inspecting presence/absence of the defects in the mask pattern, two systems, that is, an optical system and EB system are generally used as measurement systems. As inspection techniques, a system (die-to-die) for detecting differences among a plurality of chips formed on the same photo mask for exposure, and a system (die-to-database (DB)) for detecting the mask pattern formed on the photo mask for exposure and a difference from data of the mask pattern are used. The existing problem lies in that it is very difficult to clarify specification for setting an allowable difference. That is, when the specification can be clarified from a viewpoint of mask preparation, a finished product can be manufactured without correcting the defects having a value not more than a certain allowable value, and it is possible to enhance yield of mask supply. However, it is difficult to set the allowable value in situations.
To solve the problem, the masks for exposure (program defect masks) produced using a plurality of data called program defects including a defect dimension and defect generation position are prepared, a defect transfer test from the mask to wafer using exposure apparatus is carried out for each type of defect, and it is judged whether the masks could be applied for making LSI chips or not.
Moreover, a technique for determining the size of the defect having the allowable value from the judgment result has been used. In recent years, the result of the transfer test conducted on the wafer using the mask for exposure prepared by the program defect has also been reported (Proc. SPIE Vol. 3677 pl. 722–733, J. Fung Chen, et al.). Moreover, in recent years, this report has been complemented by introducing a lithography simulation technique. In simulation, the data such as the defect dimension, and generation position are changed, the transfer test is carried out, and the allowable defect dimension, and the like of the mask for exposure are obtained.
Moreover, in recent years, a technique of directly diverting a mask image obtained by an inspection apparatus to transfer simulation and judging whether the mask could be applied for making LSI chips or not has also been reported (Proc. SPIE Vol. 3677 pl. 711–720 Donald Pettibone, et al., Wafer Printability Simulation Accuracy Based on UV Optical Inspection Images of Reticle Defects). Furthermore, a technique of observing the mask for exposure in an apparatus having a resolution characteristic equal to that of an exposure optical system and inspecting the transfer of the defect on the wafer (defect transfer characteristic) has also been proposed (Proc. SPIE Vol. 3236 pl. 136–141, Fritz Ganz, et al.).
However, these have some problems.
It is difficult to generalize the technique of preparing the program defect mask, carrying out the transfer test, and evaluating the mask because of limited types of defects. Moreover, it is necessary to set the defect size to be detected, that is, so-called inspection sensitivity for each type of the defect, and this technique is remarkably complicated and specialized.
Furthermore, for the technique of judging the transfer characteristic by the simulation, even when the method of extracting a contour from the mask pattern is used, it is difficult to evaluate an influence of topography effect by a micro pattern or a “gray tone” defect whose phase or transmittance is not equal to the mask Qz(quatz) substrate nor pattern itself (opaque or attenuated).
The above-described respect will concretely be described with reference to FIGS. 3 to 11. FIG. 3 is a plan view of the mask pattern formed on the mask for exposure. As shown in FIG. 3, in the mask for exposure, a plurality of linear halftone portions 21 and space portions 22 are arranged on a substrate formed of quartz, and the like. The halftone portions 21 and space portions 22 form a line and space (L&S) pattern. The mask for exposure is assumed as its magnification factor from wafer to mask is 4×, and the mask for exposure is used to manufacture a pattern having a line width of 0.15 μm and space width of 0.15 μm on the wafer. It is assumed that a defect 23 exists in a region A of the line and space pattern on the mask for exposure.
FIGS. 4A, 4B, 4C, 5A, 5B, 5C, 6A, 6B, 6C, 7A, 7B and 7C are enlarged views showing a binarized image intensity distribution obtained by the detection optical system, which is assumed to assemble an inspection system of mask. when the size, phase, and transmittance of the defect 23 present in the region A are changed. Assumed optical conditions of the detection optical system and conditions of the mask for exposure as an object to be measured are as follows.
An inspection wavelength for use in the detection optical system=248 nm, numerical aperture (NA)=0.9, coherence factor (σ)=0.8, a halftone phase shift mask is used, and threshold value for use in binarization=0.1769. Additionally, the threshold value is a value obtained when the halftone portions do not exist, the influence of diffraction is not exerted, and an opening transmitted light of the mask for exposure is normalized as unit (1).
Additionally, the defects 23 shown in FIGS. 4A–4C, 5A–5C, 6A–6C and 7A–7C are black defects which have sizes of 100 nm, 200 nm, and 300 nm. A phase difference is 180 degrees in FIGS. 4A–4C, and 90 degrees in FIGS. 5A–5C, and the transmittance is 0% in FIGS. 6A–6C, and 12% in FIGS. 7A–7C.
FIG. 8 is a characteristic diagram showing a relation between the defect detected size by the inspection optical system (ordinate) (μm) and the dimension of the defect present in the mask for exposure (abscissa) (μm). That is, FIG. 8 shows a relation between the size of the defect obtained by the detection optical system and the size of the defect on the actual reticle (mask). In FIG. 8, -x- corresponds to FIGS. 4A–4C, -◯- corresponds to FIGS. 5A–5C, -▪- corresponds to FIGS. 6A–6C, and -Δ- corresponds to FIGS. 7A–7C.
FIGS. 9 to 11 are characteristic diagrams showing a relation between a depth of focus (DOF) (μm) (ordinate) and an exposure amount allowance (%) (abscissa), and show the deterioration of the exposure amount allowance of the resist dimension on the wafer with respect to the defect on the mask for exposure. In the figures, similarly as FIG. 8, -x- corresponds to FIGS. 4A–4C, -◯- corresponds to FIGS. 5A–5C, -▪- corresponds to FIGS. 6A–6C, and -Δ- corresponds to FIGS. 7A–7C. Moreover, in FIG. 9, the defect size on the mask for exposure is 0.1 μm. In FIG. 10, the defect size on the mask for exposure is 0.15 μm. In FIG. 11, the defect size on the mask for exposure is 0.2 μm.
Exposure conditions for use during the transfer simulation onto the wafer are hereinafter shown. FIG. 12 is a schematic perspective view of an exposure apparatus for use in the embodiment of the present invention and in a conventional example.
The exposure conditions include the exposure wavelength=248 nm, numerical aperture (NA)=0.68, coherence factor (σ)=0.75, and annular illumination with its center-shielding ratio is 2/3.
Here, as a model for obtaining a resist dimension, a technique of convoluting/integrating Gaussian function with respect to an optical image I(x), and defining the dimension with the threshold value corresponding to an exposure amount is used.                                                                         Ig                ⁡                                  (                  x                  )                                            =                            ⁢                                                I                  ⁡                                      (                    x                    )                                                  ⁢                                                                  ⁢                                  (                  x                  )                                ⁢                                  g                  ⁡                                      (                    x                    )                                                                                                                                          =                                ⁢                                                      1                    /                                          (                                                                                                                                  (                              π                              )                                                                                ·                          Δ                                                ⁢                                                                                                  ⁢                        L                                            )                                                        ⁢                                      ∫                                                                  I                        ⁡                                                  (                                                      x                            -                                                          x                              ′                                                                                )                                                                    ⁢                                              exp                        ⁡                                                  (                                                                                                                    -                                                                  x                                                                      ′                                    ⁢                                                                                                                                                  ⁢                                    2                                                                                                                              /                              Δ                                                        ⁢                                                                                                                  ⁢                                                          L                              2                                                                                )                                                                    ⁢                                              ⅆ                                                  x                          ′                                                                                                                                ⁢                                                                                                      (        7        )            
Here, the equation was calculated assuming ΔL=60 nm, and (x) represents convolution/integration (x is put in ◯ in an actual symbol).
For example, assuming that a necessary allowance is the exposure amount allowance of 10% or more, and the depth of focus (DOF) is 0.5 μm or more, as shown in FIG. 10, when the size of the defect on the mask for exposure is 0.15 μm, a defect having a phase of 180 degrees and transmittance of 0%, and a defect having a phase of 90 degrees and transmittance of 6% are accepted. However, the other defects are rejected. As shown in FIG. 8, the size of the defect detected by the corresponding inspection apparatus is 0 for the accepted defect, and is of the order of 0.08 to 0.1 μm for the rejected defect.
As shown in FIG. 11, when the size of each defect on the mask for exposure is 0.2 μm, all the defects are rejected. When these defects are seen from the viewpoint of the exposure allowance, there is little difference between the defect having a phase of 180 degrees and transmittance of 0% and the defect having a phase of 90 degrees and transmittance of 6%. However, both defect sizes are detected to be different from each other by about 20 nm by the inspection optical system.
When the transmittance and phase differ in this manner, a correlation between the allowable defect size and the size obtained by the detection optical system is deteriorated. Therefore, the allowable defect size cannot but be set to be in a strict direction. As a result, even the mask for exposure which can originally be shipped is judged to be NG, and production yield in manufacturing the mask for exposure decreased.
It is also possible to use the inspection apparatus which has a resolution characteristic (NA/λ/σ) equal to that of the exposure apparatus. In this case, the characteristics (such as aberration) inherent in the actual exposure apparatus differ in many cases, and it is difficult to remove the influence.
As described above, the defect dimension accepted in the conventional inspection technique cannot but be set to the allowable value which has a considerable allowance. As a result, it is difficult to supply the photo mask for exposure within the allowable value.